Industrial plants having containers or tanks (“tanks”) generally need to regularly measure the level of liquid(s) or other materials therein such as powders therein. There are several types of systems and techniques used for level measurement, which generally use either frequency modulated continuous wave (FMCW) or time domain reflectometry (TDR) that relies on analyzing echoes for pulsed signals to make material level measurements using time-of-flight.
For TDR-based measurements, there are contact level measurements, where a part of the system, such as a probe, contacts the material being measured, and non-contact level measurements where the level is measured without contacting the material to be measured. Non-contact methods include ultrasound, which uses high-frequency sonic (sound) waves to detect the level, and radar.
Guided wave radar (GWR) is a particular contact pulsed radar method used to measure the level of liquids or solids in a tank. GWR works by generating a stream of pulses of electromagnetic energy and propagating the pulses down a transmission line formed into a level sensing probe (or waveguide). The probe is generally placed vertically in a tank or other container and the electromagnetic pulse is launched downward from the top of the probe. The probe is open to both the air and the material(s) to be sensed in such a way that the electromagnetic fields of the propagating pulse penetrate the air until they reach the level of the material. At that point, the electromagnetic fields see the higher dielectric constant of the material. This higher dielectric constant causes a reduction in the impedance of the propagating medium, resulting in a pulse echo being reflected back toward the top of the probe. The pulse travels through the air dielectric portion of the probe at a known velocity. This allows the material level(s) on the probe to be determined by measuring the round trip travel time of the pulse from the top of the probe to the level and the echo back to the top of the probe.
When using a pulsed radar sensor (whether contact or non-contact) for measuring a first product material level in a tank, if a second material having a lower density such as an oil is also in the tank, a layer of the second material will be formed on top of the first material (e.g., water) surface creating a liquid interface. Customers are generally interested in knowing the thickness or level of the second layer, such as to allow the selective removal of the second layer, for example when there is oil on water, and one may want to selectively remove the oil.